The invention relates to a method and an apparatus for form machining the peripheral edge of spectacle lenses and of subsequently applying, if need be, a bevel, having a driven spectacle-lens holding shaft holding the spectacle lens and a cutting tool movable in a controlled manner in relation to the spectacle-lens holding shaft.
In order to carry out the form machining of the peripheral edge of spectacle lenses as quickly as possible without the risk of breaking or damaging the spectacle lenses and at the same time definitely avoid a situation in which the spectacle lens clamped in place in the spectacle-lens holding shaft slips through, it is known according to DE 196 16 536 A1 of the same applicant for the force with which the cutting tool is guided against the spectacle lens to be increased in a controlled manner as a function of the respective radius, from a large radius to a small radius, in the bearing region of the spectacle lens touching the cutting tool. In this way, the cutting pressure can be set in such a way that, at a large spectacle-lens radius touching the grinding wheel, it is just so large that a situation in which the spectacle lens clamped in place in the spectacle-lens holding shaft slips through is just avoided, in which case the cutting pressure can be increased with decreasing radius, and this increase in turn, on the one hand, depends on the permissible instantaneous torque exerted on the spectacle lens by the cutting pressure, but, on the other hand, must not become so large that the spectacle lens is damaged or even destroyed as a result.
In this known method, the cutting pressure can be controlled by means of a data volume of a computer, which also controls the form machining of the spectacle lens by means of this data volume. In particular, the cutting pressure can be controlled by varying the torque transmitted by the servomotor moving the cutting tool relative to the spectacle-lens holding shaft, in which case the variation in the torque can be carried out either by means of a torque-controlled servomotor or by means of a torque-controlled coupling between the servomotor and the cutting tool which can be fed in.
The method described above and the associated apparatus have proved successful; however, a relatively complicated control is required for this.
It is known from DE 196 32 340 A1 to measure the torque of the drive motor of a spectacle-lens holding shaft and/or of a grinding-wheel spindle in a spectacle-lens edging machine and to use it to control the cutting force. A variation in the cutting force is achieved by influencing a vertical-adjustment device for the spectacle-lens holding shaft, which is activated in such a way that a pin descends, specifically in such a way that a predetermined grinding pressure is obtained in the contact region between the raw glass and the grinding wheel.
The method described above has proved successful, but can be simplified if the control for the shaping of the spectacle lens is separated from the control of the applied pressure.
The problem underlying the invention is likewise to provide a method and an apparatus for form machining the peripheral edge of spectacle lenses and for subsequently applying, if need be, a bevel, with which a situation in which the spectacle lens clamped in place in the spectacle-lens holding shaft slips through is definitely avoided, and with which the form machining of the peripheral edge of spectacle lenses can be carried out as quickly as possible without the risk of breaking or damaging the spectacle lenses, in which case this method and the apparatus are to be of simple design or construction and are not to require a complicated control.
Starting from this definition of the problem, it is proposed in a method of the type mentioned at the beginning that the torque of the spectacle-lens holding shaft be measured, that form machining of the peripheral edge of the rotating spectacle lens be carried out with a predeterminable, adjustable force of the cutting tool if and as long as the torque is less than or equal to a predetermined, set value, and that the cutting-tool force acting on the spectacle lens be reduced by reducing the rotational speed of the spectacle-lens holding shaft, if need be until it stops, if and as long as the torque exceeds the predetermined value.
As a result, the cutting force acting on the spectacle lens clamped in place in the spectacle-lens holding shaft always remains below a predeterminable value at which the spectacle lens definitely does not slip through. This method has the advantage that it can be integrated in a spectacle-lens edging machine independently of the control of the form machining, if need be even by retrofitting. For example, the method according to the invention does not depend on whether the form machining of the spectacle lens is effected in a CNC-controlled manner or by copy grinding with the aid of a form wheel.
The measurement of the torque of the spectacle-lens holding shaft can be carried out in different ways, for example by measuring the torque of the drive motor of the spectacle-lens holding shaft. This torque is proportional to the power input, so that the measurement of the torque can be achieved by measuring the power input of the drive motor. Likewise, the torque of the drive motor can be measured at the housing of the drive motor if this housing is mounted on an elastic support so as to be slightly rotatable and the displacement of the elastic support is measured.
The rotational speed of the spectacle-lens holding shaft may be controlled either in such a way that it is reduced in proportion to the amount by which the predetermined value of the torque is exceeded or by the spectacle-lens holding shaft being stopped for as long as a predetermined value of the torque is exceeded.
According to the invention, the apparatus for form machining the peripheral edge of spectacle lenses and for subsequently applying, if need be, a bevel, having a driven spectacle-lens holding shaft holding the spectacle lens and a cutting tool movable in a controlled manner in relation to the spectacle-lens holding shaft, has a measuring device for measuring the torque of the spectacle-lens holding shaft, and a control device for the force, acting on the spectacle lens, of the cutting tool for reducing this force by reducing the rotational speed of the spectacle-lens holding shaft, if need be until it stops, if and as long as the torque exceeds a predetermined value.
The control device can act on the drive of the spectacle-lens holding shaft for reducing the rotational speed of the spectacle-lens holding shaft, if need be until it stops.
The measuring device for measuring the torque preferably has an elastic support of the rotatably mounted housing of the drive motor for the spectacle-lens holding shaft, in which case a position encoder interacting with the control device may be arranged on the support, and the control device brings about a reduction in the rotational speed of the spectacle-lens holding shaft in proportion to the amount by which a predetermined displacement of the position encoder is exceeded.
An especially advantageous embodiment of the apparatus according to the invention is obtained if the elastic support is prestressed in such a way that that a movement does not occur until the prestress is exceeded, i.e. until the predetermined torque is exceeded, and in which an electrical contact interacts with the elastic support, this electrical contact interrupting the power supply to the drive motor when the prestress is exceeded.
The method according to the invention and the apparatus can be combined with the method described in DE 196 16 536 A1 in such a way that, on the one hand, the cutting pressure is increased in a controlled manner as a function of the respective radius, from a large radius to a small radius, in the bearing region of the spectacle lens touching the cutting tool and, on the other hand, the rotational speed of the spectacle-lens holding shaft is reduced, if need be until it stops, if and as long as the torque exceeds the predetermined value.